Trees change their colours now.
Chlorophyll produces energy in a plant and gives it its green summer colour. When after summer there is less light and the days and nights get colder, cork cells form in the veins of leaves. Thus the transport of nutrients and water in the leaf is interrupted and it cannot make any more new chlorophyll. When there is no more chlorophyll in stock because it is all used up, the leaf loses its green colour. Other pigments, especially carotenoids who are yellow or orange, become visible. They have been there all summer, but only now you can see their colour.
Some leaves of an Norway maple (Acer platanoides) are fallen in a shallow stream. In the green leaf below chlorophyll is still visible between the veins, elsewhere other pigments are dominating.
The changes in the metabolism in the leaf, diminishing light, and the fall in temperatures provoke chemical processes which result in other sets of pigments.
Here, in a Blackberry bush (Rubus fruticosus), red anthocyanids have emerged with the first frost.